The present invention relates to a polarization-selective elements with a substrate-mode grating pair structure.
Polarization-selective elements have many applications, such as in switching cells of optical multistage networks and in the polarization-sensing device of the optical pickup head for a magneto-optic data storage system. For switching cells, the devices consist of polarization beam splitters and electro-optic half-wave plates, and electric signals are used to control optical signal paths. For the optical head, the polarization-selective element is used to detect the rotation of the polarization of the beam reflected from the magneto-optic disk medium. Conventionally, in FIG. 1, a prior polarization beamsplitter is shown, wherein item 10 is an incident beam and item 13 is a polarization beamsplitter, and numbers 11 and 12 indicate the s-polarized and p-polarized fields, respectively. The prism cube is used as the polarization beam splitting element. In 1990, holographic elements (U.S. Pat. No. 4,946,253) with a high diffraction angle were designed to replace those prism devices.
Referring now to FIG. 2, number 27 refers to the substrate. An input polarization-independent coupling grating 23, a polarization-selective grating 24, an output s-field coupling grating 25 and the output p-field coupling grating 26 are provided. An incident beam 20 with s-polarization and p-polarization fields is diffracted by the input polarization-independent coupling grating 23 in the substrate, then propagates through the substrate to the polarization-selective grating 24 where the s-polarization beam 21 is split from the p-polarization beam 22. These two beams separately propagate through the substrate at angles beyond the critical angle and with total internal reflection to the output s-field coupling grating 25 and output p-field coupling grating 26, and then is normally coupled out with conjugate diffraction at directions parallel to each other. For this transmission-type phase volume hologram, the relation between the diffraction efficiency for an s-polarized field .eta..sub.s and the efficiency for a p-polarized field .eta..sub.p with the same Brag reconstruction condition depends strongly on the diffraction angle and is expressed as EQU .eta..sub.s,p =sin.sup.2 .nu..sub.s,p ( 1)
and the modulation parameters, .nu..sub.s (s field) and .nu..sub.p (p field), are given, respectively, as ##EQU1## where .lambda. is the reconstruction wavelength, d is the medium thickness, n.sub.1 is the index modulation, and .THETA..sub.r1 and .THETA..sub.r2, respectively, are corresponding angles of the reconstruction and diffracted beams in the hologram medium.
From the diffraction efficiency formulas in Eqs. (1)-(3), it can be seen that when the value of .nu..sub.s is .pi./.sub.2, then .eta..sub.s =100%, and if the diffraction angle, .vertline..THETA..sub.r2 -.THETA..sub.r1 .vertline., is also 90.degree., then .nu..sub.p =0 and .eta..sub.p =0%. In this case, a highly polarization-selective element can be obtained. However, because of the high diffraction angles (90.degree.) this element needs additional input and output couplers to form a complete device. This type of element has been investigated, and a typical structure is shown in FIG. 2. In this configuration, for the polarization-selective grating 24, .THETA..sub.r1 =45.degree., .THETA..sub.r2 =-45.degree., and n.sub.1 =0.016 with the film thickness of 17 .mu.m, we have .eta..sub.s =100% and .eta..sub.p =0%. For the input polarization-independent coupling grating 23, .THETA..sub.r1 =0.degree., .THETA..sub.r2 =45.degree., and n.sub. 1 =0.022, we have .eta..sub.s =.eta..sub.p =93%; for the output s-field coupling grating 25, .THETA..sub.r1 =-45.degree., .THETA..sub.r2 =0.degree., and n.sub.1 =0.019, we have .eta..sub.s =100%; for the output p-field coupling grating 26, .THETA..sub.r1 =45.degree., .THETA..sub.r2 =0.degree., and n.sub.1 =0.027, we have .eta..sub.p =100%.
Theoretically the maximum transmission efficiency for both s and p channels is only 93%. In addition, four different grating angles and index modulation values make the complete component complicated and difficult to fabricate.
A new structure is disclosed having a smaller diffraction angle to perform a highly polarization-selective procedure that provide a higher transmission and efficiency and a much simpler fabrication process.